The IEEE 802.16 Working Group on Broadband Wireless Access Standards develops formal specifications for the global deployment of broadband Wireless Metropolitan Area Networks. Although the 802.16 family of standards is officially called Wireless MAN, it has been dubbed WiMAX by an industry group called the WiMAX Forum. IEEE 802.16e-2005 (formerly known as IEEE 802.16e) is in the lineage of the specification family and addresses mobility by implementing, e.g., a number of enhancements including better support for Quality of Service and the use of Scalable OFDMA. Like OFDM, OFDMA transmits a data stream by dividing the data stream over several narrow band sub-carriers (e.g. 512, 1024 or even more depending on the overall available bandwidth (e.g., 5, 10, 20 MHz) of the channel) which are transmitted simultaneously. As many bits are transported in parallel, the transmission speed on each subcarrier can be much lower than the overall resulting data rate. This is important in a practical radio environment in order to minimize effect of multipath fading created by slightly different arrival times of the signal from different directions. In general, the 802.16 standards essentially standardize two aspects of the air interface—the physical layer (PHY) and the media access control layer (MAC).
The WiMAX Forum has defined an architecture for connecting a WiMAX network with other networks, such as networks complying with IEEE 802.11and cellular networks, and a variety of other aspects of operating a WiMAX network, including address allocation, authentication, etc. FIGS. 1A and 1B show non-limiting examples of WiMAX networks, and it should be understood that the arrangement of functionalities depicted in FIGS. 1A and 1B can be modified in WiMAX and other communication systems.
As depicted in FIG. 1A, the network 100A includes base stations (BSs) 102, 104, 106, 108 that respectively transmit and receive radio signals in geographic areas called “cells”, which typically overlap to some extent as shown. Subscriber stations (SSs) 110, 112 are located in the cells and exchange radio signals with the BSs according to the WiMAX air interface standard. An SS is typically either a mobile SS (MS) or a fixed SS, and it will be understood that a network can include many cells and many SSs. In FIG. 1A, the BSs communicate with and are controlled by Access Service Network (ASN) Gateways (G/Ws) 114, 116 that also communicate with each other, and with other core network nodes and communication networks (not shown), such as the public switched telephone network and the internet. SSs, such as SSs 110, 112, can be organized into groups for paging, as described in more detail below.
FIG. 1B depicts a WiMAX network 100B that also includes BSs 102, 104, 106, 108 and SSs 110, 112 as in the network 100A. The network 100B is more decentralized than the network 100A in that, in FIG. 1B, the BSs communicate with each other directly through a suitable routing network 118 that also communicates with other core network nodes and communication networks (not shown).
According to one mode of IEEE 802.16,the downlink (DL) radio signals transmitted by the BSs are orthogonal frequency division multiple access (OFDMA) signals. In an OFDMA communication system, a data stream to be transmitted by a BS to a SS is portioned among a number of narrowband subcarriers, or tones, that are transmitted in parallel. Different groups of subcarriers can be used at different times for different SSs. Because each subcarrier is narrowband, each subcarrier experiences mainly flat fading, which makes it easier for a SS to demodulate each subcarrier.
The DL radio signals and uplink (UL) radio signals transmitted by the SSs are organized as successions of OFDMA frames, which are depicted in FIGS. 2A, 2B according to a time-division duplex (TDD) arrangement in the IEEE 802.16e standard. FIG. 2B magnifies FIG. 2A and shows the format of the DL and UL subframes in more detail than in FIG. 2A. In FIGS. 2A, 2B, time, i.e., OFDMA symbol number, is shown in the horizontal direction and subchannel logical number, is indicated by the vertical direction. A subchannel is a pre-defined group of OFDM subcarrier frequencies that may be contiguous or non-contiguous. FIG. 2B shows one complete frame and a portion of a succeeding frame, with each DL subframe including sixteen symbols and each UL subframe including ten symbols, not counting guard symbols.
Each DL frame 200 starts with a preamble signal that includes a known binary signal sent on every third OFDM tone or subcarrier, as depicted by FIG. 3. The range of subcarriers shown in FIG. 3 is numbered 0, 3, 6, . . . , 1701,but a preamble can use fewer than that many subcarriers.
As seen in FIGS. 2A, 2B, each frame's preamble is followed by a DL transmission period and then an UL transmission period. According to the WiMAX standard, the preamble signal is sent in the first OFDM symbol of a frame, which is identified by an index k in FIG. 2B, and is defined by the segment, i.e., one of the three sets of tones to be used, and a parameter IDCell, which is the transmitting cell's identification (ID) information. A SS uses the preamble for initial synchronization of its receiver to the BS (the network), and to determine the location of a frame control header (FCH), which is among the first bursts appearing in the DL portion of a frame. A SS also uses the preambles in signals transmitted by neighboring BSs to synchronize to them for purposes of measurement for handover from one cell to another.
The FCH gives information on the DL signal parameters, including a DL map message (DL-MAP), which is a medium access control (MAC) message that defines DL allocations for data, and parameters relevant for reception of the signal. The DL-MAP may be followed by an UL map message (UL-MAP), which provides UL allocations for data, and other parameters relevant for transmission of signals from an identified SS. With the assignments in time and frequency from the DL-MAP, an identified SS can receive the data in the particular location. Similarly, it can identify assignments in time and frequency on the UL-MAP, and transmit accordingly. FIGS. 2A, 2B also show a transmit/receive transition gap (TTG) interval and a receive/transmit transition gap (RTG) interval, which are used by the BS and SS to switch from transmit to receive and vice versa.
FIG. 2A also illustrates how a BS pages an SS operating in idle mode, showing the relationship between paging cycles, paging offset, BS paging interval, and OFDMA frames. Only two paging cycles are shown in FIG. 2A. An SS “listens” for a page message from the BS during only a portion of a paging cycle, and the location of that paging interval is determined by a paging offset from the start of the paging cycle. A paging message can span several OFDMA frames, which the SS needs to demodulate to read the entire message.
Thus, while a SS is idle, the SS periodically turns on its baseband processing unit, which includes a fast Fourier transform (FFT)-based demodulator and decoder, even when there are no paging messages for it and no system configuration changes/updates. The SS first synchronizes with the preamble and reads the FCH, and it then reads the DL-MAP to look for the location and the format of a broadcast connection identifier (CID). If the DL-MAP shows a broadcast CID, the SS demodulates that burst to determine whether there is a BS broadcast paging message (MOB_PAG-ADV).
Most of the time, there is no paging message and no action required by the SS, but during each paging interval, an SS has to be fully “awake”, which is to say, its receiver has to be powered-up, for a number of OFDMA frames, using electrical power and draining a battery over time. In addition to MOB_PAG-ADV messages, changes in channel descriptors or broadcast system updates can trigger an idle SS to stay on for updating the system parameters or reading other coming messages.
A quick paging mechanism that can reduce the negative effects of the conventional paging mechanism is desirable for current and future versions of the WiMAX standards. In such a quick paging mechanism, a simple signal would indicate to a group of one or more SSs that a paging signal exists in a subsequently transmitted signal block. In effect, quick paging is a first part of a two-part paging process. During the first part, a shorter more ambiguous message is addressed to a group SSs to quickly inform them that there is a paging message intended for at least one member of the group in subsequent frames. On receipt of this quick paging message, members of the addressed group also monitor the information received in the second part corresponding to the actual paging message. Based on the paging message, which is not ambiguous, the SS can determine if it is being paged. SS's that are not addressed by the quick paging message go back to sleep and do not listen to the paging message, thus saving battery life.
Currently, a new standard for mobile radio broadband is being developed in IEEE 802.16m. This standard is required to be backwards compatible to mobile WiMAX system, i.e., IEEE 802.16e-2005,but may also have a non-backwards compatible mode. In developing IEEE 802.16m, a proposal has been made for a quick paging mechanism that is described in IEEE C802.16m07/217,“Wake-up Signal for 802.16m OFDMA Idle Mode” (Nov. 7, 2007). If an SS decodes the quick paging signal correctly, the SS needs to listen to the conventional paging signal; otherwise, the SS can go back to “sleep”, thereby saving its resources, such as battery power. But this proposal either takes resources away from the available system resources, thereby reducing system capacity, or occupies transmit and receive gaps in a time-division duplex (TDD) version of the system, that may lead to compatibility issues among different device implementations.
The design of the synchronization channel (SCH) for IEEE 802.16m has not yet been completed. In backwards compatible mode, the preamble (i.e., the legacy synchronization channel) would have to be present, but could be sent in addition to a new IEEE 802.16m-specific SCH. In a non-backwards compatible mode, the legacy preamble would not have to be present. It is possible to use a non-hierarchical SCH, or a hierarchical SCH based on a primary SCH (P-SCH) used for initial acquisition and common to all sectors and cells and a secondary SCH (S-SCH) used for fine synchronization and carrying sector/cell ID information. A working assumption in the 802.16m system description document IEEE 802.16m-08/003r4,the disclosure of which is incorporated herein by reference, is that either the P-SCH (hierarchical case) or the SCH (non-hierarchical case) is mapped onto every other sub-carrier in the frequency domain. Similarly, the S-SCH (hierarchical case) is mapped onto every Nth sub-carrier (the value N is not specified but could be a number such as 3 for example). For the hierarchical case, it is alternatively possible to reuse the legacy preamble as S-SCH (hierarchical case). The legacy preamble is sent on every third sub-carrier.
If the entire system bandwidth in IEEE 802.16m is used for the SCH, then SSs that are not capable of receiving that entire bandwidth can not utilize the full SCH. As a result, such a SS will suffer reduced synchronization accuracy or may even be incapable of synchronization. On the other hand, if a sub-band of the full system bandwidth is used for the SCH, frequency division multiplexed (FDM) with data and/or control signaling transmitted outside the sub-band used for the SCH, then the OFDMA symbol can not be made periodic over a fraction of the OFDMA useful symbol time. That lack of periodicity complicates the synchronization process in the subscriber station adding delay to the process and expense to the subscriber station. With a periodic synchronization signal, all that the subscriber station needs to look for is a signal or code that repeats multiple times. It is not necessary for the subscriber station to know the value of a particular code or to detect and match/correlate the detected code with some other specific code in order for synchronization to be obtained. As long as the subscriber station detects that the same signal/code is being repeated with the periodicity of every Nth subcarrier, the subscriber station knows that it is a synchronization signal and can quickly and cost-effectively obtain synchronization with the base station. However, if every Nth subcarrier is utilized within a sub-band of the entire available frequency band for a symbol transmission period, then the subscriber station will not receive a periodic synchronization signal over the OFDMA useful symbol time. This means that the benefits associated with a periodic synchronization signal are not obtained.
The inventors recognized that a problem with a quick paging solution that is based on or designed for the legacy preamble for synchronization is that the legacy preamble may not be present in the non-backwards compatible mode of 802.16 m. In that case, the non-backwards compatible mode of 802.16 m would miss out on the benefits of quick paging. Also, if a quick paging indicator is not transmitted simultaneously with the SCH, then the SS needs to stay awake for a longer time which reduces its battery life. It also requires the SS to perform one or more extra steps, such as reading the frame control header to acquire frame configuration parameters, before being able to read the paging indicator.
Thus, if a quick paging signal is not sent simultaneously with the SCH, an idle subscriber station must stay awake (i.e., the receiver is powered on) both during the SCH transmission (to achieve synchronization and thus enable quick paging signal detection) and during the quick paging signal transmission. The longer the subscriber station needs to stay awake, the more power is consumed and the faster the battery life is reduced.